The present invention relates generally to containers for storing hazardous substances. More specifically, the invention relates to containers which store radioactive substances which are easily and effectively sealable while being able to resist access to the substances therein.
With the widespread use of radioactive substances in the medical and scientific fields there are frequent requirements to package, store and transport them between facilities and to repositories. Radioactive substances, of course, present significant hazardous conditions and they must be safely contained and transported. The physical transfer of the radioactive materials involves a rigorous packaging so as to preclude spread of loose radiocontaminants and/or unauthorized tampering with the material. Given the nature of the hazards associated with radioactive materials, the government has established regulations governing the construction and transportation of containers. The regulations, such as 10 CFR 20, establish exacting methods for human protection which often require expensive and time consuming packaging procedures. Compliance with these requirements is, of course, costly.
However, certain government regulations provide somewhat less stringent and costly packaging criteria for the encapsulation of the radioactive material; see, for example, 10 CFR 70 and 49 CFR 173.403(z). These criteria relate to containing small quantities of radioactive materials which must be so encapsulated that there cannot be emission or spread of radioparticulate contamination. To qualify for such special status, the radioactive materials cannot be directly accessed without destruction of the container. Moreover, government regulations such as those in 49 CFR 173.403 also specify that the container must be empirically provable as: impervious to gas influence or effluence at one atmosphere pressure; being able to retain its integrity after ten minutes subjugation to a temperature of 1472.degree. Fahrenheit; being able to withstand repeated drops onto a unyielding surface from a preselected height; and, having unyielding integrity after an iron bar of preselected weight free-falls from a preselected height onto each of the container's surfaces. When at least these special status requirements are satisfied, small amounts of radioactive materials are permitted to be stored and transferred by and between users with much less costly packaging. It is, therefore, highly desirable to qualify for such special status.
Numerous devices have been used for packaging materials. Not all are, however, usable for successfully sealing radioactive materials which meet exacting government guidelines. For example, U.S. Pat. Nos. 1,702,878 and 1,431,911 describe enclosures incorporating pipe thread or similarly tapered thread binding in both pitch and depth of thread, to make a tight seal. Tapered seals have not been found to be leak-tight when subjected to rigorous government standards required for the transportation of radioactive materials. For instance, it has been determined that conventional tapered threads are not leak-tight to helium after heating to 1472.degree. F. It was concluded that any threaded connection would leave a labyrinthian passage for gas leakage after heating. Accordingly, enclosures such as those noted above and described in U.S. Pat. Nos. 2,611,507; 4,109,820 and 4,711,365 were not considered starting points for providing a capsule or container which satisfies the government standards for the noted special form containers.
Other similar type closures for sealing material use, for example, solder as a thread sealant. However, the use of solder as a thread sealant precludes any container so sealed from retaining leak tightness to the helium after heating to 1472.degree. F. This is because all normal solder that is compatible with the metals of these constructions will melt and flow from the joint at much lower temperatures than required by the government. Moreover, known prior art patents do not disclose an internal sealant plug in conjunction with a machined threaded enclosure member. U.S. Pat. No. 4,754,894 and U.S. Pat. No. 4,738,388 both describe closures for massive structures for large quantities of radioactive or hazardous substances. The types of container constructions disclosed, however, would not satisfy conditions required by the government for the noted special form status.